Ready to catch some interstellar rays this summer?

Don't blame the sun for your sunburn this upcoming summer - well, not all of it.

Astrophysicists have calculated how much of the damage is being caused by stars not just outside our solar system, but other galaxies - one hundred-billionth.

"Most of the photons of light hitting us originate from the sun, whether directly, scattered by the sky, or reflected off dust in the Solar System," says International Centre for Radio Astronomy Research astrophysicist Professor Simon Driver, who led the study.

"We're also bathed in radiation from beyond our galaxy, called the extra-galactic background light. These photons are minted in the cores of stars in distant galaxies, and from matter as it spirals into supermassive black holes."

Around 10 billion photons from outside the Milky Way hit us every second when we're outside, but it would take trillions of years for them to cause any damage. And they're far outnumbered by the number that hit us from our own sun every second - 1 sextillion, or 1,000,000,000,000,000,000,000.

Where your tan comes from (supplied)

The universe isn't just trying to make our sunburn that little bit worse, though - much of the energy carried in star-hopping ultraviolet light is absorbed by intergalactic dust.

"The galaxies themselves provide us with a natural suntan lotion with an SPF of about two," says Professor Rogier Windhorst of Arizona State University.

It's little comfort for those suffering sunburn. Instead of relying on the universe's lint, Consumer NZ recommends using a sunscreen with an SPF - sun protection factor - of 50.

The findings come as part of a larger study into how the universe went from having a smooth distribution of energy and matter in its early days, to the several-billion light years across mess it is now.